(1) Field of the Invention
The present invention relates to a speed measuring device. More particularly the invention is a device to accurately measure the speed of an array being towed through the water at operating depth.
(2) Description of the Prior Art
Towed array self-noise is directly related to the speed at which the array travels through the water. Therefore, the accuracy of the array speed measurement is a factor in the accuracy of the self-noise measurement. Towed array speed measurements at sea are usually determined by speed sensors on board the towing vessel. The speed of the towed array is assumed to be equal to the speed measured at the ship. However, because of the non-uniformity of currents between the ocean surface and bottom, a change in the direction of tow can cause the array speed and consequently the self-noise to vary although ship speed is held constant. The uncertainty in the array speed measurement affects the range of error for the array self-noise measurement. Measuring the speed at array depth improves array speed accuracy, which translates to an improvement in the accuracy of the towed array self-noise measurement.
One prior art system that did not rely on the ship's speed for determining the speed of an underwater device was described by Peyton in U.S. Pat. No. 4,324,135. Peyton, in order to find the speed of fishing gear trolled from a boat, teaches the use of a magnetic rotor affixed to a paddle in a towed device for sensing the speed of rotation of the paddle. The rotor opens and closes a switch in an electrical signal thereby providing pulses that are converted to acoustic signals. The acoustic signal is transmitted to a receiver unit on the ship. This arrangement for optimum performance requires special mounting of the acoustic sensor in the receiver unit as the acoustic sensor has a tendency to pitch and roll with respect of the transmitting unit. In addition the acoustic signal is transmitted a relatively long distance.